Apparatus and method for heating simultaneously with microwaves of two widely different frequencies

ABSTRACT

A heating apparatus of the type which utilizes microwaves as the source of heating energy comprising an oven having a cavity therein in which food or other material to be heated is placed while being irradiated with microwave energy from at least two different sources producing microwaves of different frequencies. The microwaves of different frequencies are fed into the oven cavity through walls of the oven which are perpendicular to each other, thus markedly reducing the feedback of microwave energy of higher frequency into the source of microwaves of lower frequency and thereby lengthening the life of the latter source. Method of heat treating material having a relatively high dielectric constant by irradiating the material with two or three different frequencies of microwaves, each different frequency being coupled to a side wall of an oven perpendicular to the wall or walls through which the other frequency or frequencies are coupled to the oven.

United States Patent 11 1 Kegereis et al.

1451 Apr. 23, 1974 APPARATUS AND METHOD FOR HEATING Primary Examiner-J. V. Truhe SIMULTANEOUSLY WITH MICROWAVES A istan ExaminerHugh D. Jaeger OF TWO WIDELY DIFFERENT Attorney, Agent, or Firm-Edward J. Kelly; Herbert FREQUENCIES Berl; Charles C. Rainey [75] Inventors: James E. Kegereis, Lake Hiawatha,

N.J.; John T. Perry, Jr., Cincinnati, [57] ABSTRACT Ohio; Ernest Kenyon Medfield A heating apparatus of the type wh1ch ut1l1zes m1cro- Mass waves as the source of heating energy comprising an oven having a cavity therein in which food or other Asslgheei The Uhlted states of America as material to be heated is placed while being irradiated represented h) the Secretary of the with microwave energy from at least two different I Army, W s g sources producing microwaves of different frequen- [22] Filed: Dec. 6, 1972 cies. The microwaves of different frequencies are fed into the oven cavity through walls of the oven which [21] APPI N04 312,437 are perpendicular to each other, thus markedly reducing the feedback of microwave energy of higher fre- 52 US. Cl. 219/1055 quency into the Source of microwaves of lower 51 1m. 01. H05b 9/06 quency and thereby-lengthening the life of the latter 58 Field of Search 219/1055 Source Meflwd of heat "eating material having a relatively high dielectric constant by irradiating the mate- [56] References Cited rial with two or three different frequencies of micro- UNITED STATES PATENTS waves, each different'frequency being coupled to a s1de wall of an oven perpendicular to the wall or walls 2,909,635 10/1959 Haagensen 219/1055 through which the other frequency or frequencies are 2,593,067 4/1952 Spencer 219/1055 Cou led to the Oven 3,493,708 2/1970 Johnson 219/1055 p 4 Claims, 1 Drawing Figure t 3 ill PATENTEDAPR 18H LID APPARATUS AND METHOD FOR HEATING SIMULTANEOUSLY WITH MICROWAVES OF TWO WIDELY DIFFERENT FREQUENCIES The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates to a microwave heating apparatus and method, and more particularly to such an apparatus and-method as may be applied to the heating of food, especially for cooking and sterilizing food.

In the past, it has been customary in the electronic heat processing of food to employ one or, in some circumstances, two sources of microwaves. Where only one source of microwaves was employed, they were generally of too high a frequency to heat relatively thick products throughout and within a reasonable length of time or of too low a frequency to do a good job of browing the exterior of the food by heating at or near the surface. In order to overcome this deficiency in the processing of foods by microwave heating, it has been proposed that a source of relatively higher frequency microwaves (e.g. about 2,450 megahertz) and a source of relatively lower frequency microwaves (e.g. about 915 megahertz) be employed either conjointly or sequentially in the microwave heat processing of foods of substantial masses and that the input means for feeding these microwave frequencies into the oven be mounted in or on the same interior wall of the oven or other apparatus and preferably separated by a baffle, in the case of antennae as disclosed by Kamide U. S. Pat. No. 2,895,828, to reduce the amount of feedback of microwave energy from the source of higher frequency and shorter wavelength into the other source. Such feedback must be maintained as low as possible to prevent sudden burning out or early wearing out of the magnetron or other means for producing the micrwaves of lower frequency and longer wavelength fed into the apparatus.

We have found, however, that microwaves behave much in the same way that visible light waves behave 1 and that when both sources of microwave energy are mounted in the same wall of an oven or other heatprocessing apparatus, there is a tendency for the microwave energy from each source to be reflected from the opposing wall toward the wall from which it was radiated and thus to be fed back into the other source despite the presence of a baffle between two antennae or the like. This is especially true of the microwaves of higher frequencies. Furthermore, such baffles severely reduce the usable space within an oven, thus necessitating much larger ovens or heat processing tunnels to accommodate food products being processed in such apparatus.

It is accordingly an object of the present invention to provide apparatus and a method for heat processing foods or other materials by means of microwave energy substantially uniformly with a higher frequency source of microwave energy and a lower frequency source of microwave energy while maintaining substantially as long a life in each of two magnetrans or other means for producing the microwaves and particularly in the source of microwaves of lower frequency as when only one source of microwaves is being employed and while maintaining maximum availability of usable space within the apparatus in which the heat processing of the foods or other materials is carried out.

Other objects and advantages will be apparent from the following description of one embodiment of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

SUMMARY The objects of the invention are accomplished by providing in a microwave heat processing apparatus at least two, or as many as three, sources of microwave energy of differing frequencies and feeding the microwaves of differing frequencies into an oven or other cavity through sidewalls which are mutually perpendicular so that the microwaves of one frequency are radiated in a generally forward direction toward an opposing sidewall from which microwaves are not being fed into the oven or other cavity. This arrangement of the microwave energy sources in relation to each other and the sidewalls which bound the cavity in which microwave heating is carried out minimizes feedback of microwaves from one source of microwaves of higher frequency and shorter wavelength into another source of microwaves of lower frequency and longer wavelength. Normally, there is no danger of feedback of any consequence from a source of microwaves of lower frequency and longer wavelength into a source of microwaves of higher frequency and shorter wavelength regardless of the relative locations of the sources. The net result of the arrangement of the sources of microwaves of differing frequencies in accordance with the present invention is to make possible simultaneous deep and shallow heating of foods or other materials having relatively high dielectric constants and, therefore, subject to microwave heating, while avoiding abnormally rapid burnout or excessively short life expectancy in one or more of the microwave energy sources, the latter effects being associated with or caused by feedback of microwave energy from one source into another source, which may result in overheating of the latter source.

Theoretically, in a six-sided oven defining a rectangular parallelepiped having substantially rectangularly shaped sidewalls and top and bottom walls, it would be possible to employ microwave sources of three different frequencies while still accomplishing the objectives of the present invention. However, for most purposes, particularly in the heat processing of food products, two sources will be sufficient for the purpose of heating even relatively large masses of food both deeply and su perficially so as to result in cooking of the food throughout its mass in orfe step so as to make the cooked food acceptable to most people while not unduly shortening the life expectancies of the microwave energy sources.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is to be understood that for the purposes of the specification and claims the term microwave refers to very short electromagnetic energy waves having wavelengths of between about 1 meter and about 1 millimeter and frequencies of at least about 300 megahertz (MHz) and up to as high as 300,000 MHz. Suitable sources of such microwave energy include crossed field magnetron oscillators, similar to those employed for radar, klystrons, and high power vacuum triodes. The common frequencies of operation allocated by the Federal Communications Commission for microwave heating purposes are 915 and 2,450 MHz. However, it is to be understood that the invention is applicable to a wide range 'of frequencies and wavelengths as indicated above.

The FIGURE is a schematic representation, partially in section, of the invention.

Referring to the drawing, reference numeral designates a substantially rectangular parallelepipedshaped oven suitable for the introduction thereinto of a foodstuff or other material requiring heat processing. An opening 11 in one of the sidewalls of the oven'provides access to the interior or cavity of the oven by way of a suitable closure member for the opening, such as door 12. Handle 13 is provided on the door for the opening or closing thereof. A container 14 is normally located within the oven serving to hold a food 15 or other material of relatively high dielectric constant which is to be heated by means of microwave energy.

A magnetron 16 provides electromagnetic energy having a frequency of 2,450 MHz. which is coupled to the oven 10 by waveguide sections 17 and 18 and horn 19 connected to the oven through the top wall 20. A second magnetron 21 provides electromagnetic energy having a frequency of 915 MHz. which is coupled to the oven 10 by waveguide sections 22, 23, 24, and 25, by antenna 26, and window 27 in side wall 28 of the oven. Window 27 is transparent to microwaves of 915 MHz-frequency, whereas the side walls and top and bottom walls of the oven and the waveguides are opaque to microwaves of either 915 or 2,450 MHz. frequencies.

It is necessary for the purpose of the invention that the walls of the oven through which the microwave energies of different frequencies are coupled to the oven be mutually perpendicular to minimize feedback of energy by reflection or radiation of the energy from one source into the other source. This is particularly important for preventing the feedback of energy of 2,450 MHz. frequency into the source of energy of9l5 MHz. frequency. Thus, instead of being coupled to the oven through sidewall 28 and top wall 20, the two different frequencies could be coupled to the oven through any of the walls of the oven which are perpendicular to each other, provided none of these walls is opposite another wall coupled to a source of microwave energy.

In the operation of the apparatus, the two sources of microwave energy of different frequencies will be preferably operated simultaneously after food or other material to be heated is placed in the oven and the door closed. The microwave energy of each frequency is radiated in a substantially forward direction from the sidewall through which its source is coupled to the oven. This saves time over sequential heating in the same or different ovens, which has been done in the past in order to heat from the center outwardly with lower frequency microwavesand from the outside inwardly with higher frequency microwaves in separate steps. It also minimizes feedback of energy of one frequency into the source of energy of the other frequency.

Although the invention has been illustrated with two different sources of microwave energy coupled to an oven, it is to be understood that three different sources of microwaves of different frequencies may be employed, if desired, the essential requirement being that the sources of microwave energy be coupled to the oven through walls of the oven which are mutually perpendicular so that the energy from any one of the three sources will not be reflected in any appreciable amounts toward a wall through which either of the other sources is coupled to the oven.

Although in the foregoing description of the invention frequencies of the microwave energy used in connection with the specific embodiment of the invention were 915 HMz. and 2,450 MHz., it will be understood by those skilled in the art that other frequencies in the microwave range of the electromagnetic spectrum may be employed. The important consideration is that the relatively lower frequency should be sufficiently low in the microwave frequency band to be characterized by a preferential penetrating or deep heating effect, particularly with larger or thicker bodies of food, while the relatively higher frequency should be sufficiently high in the microwave frequency band to be characterized by a preferential surface or shallow heating effect, which is needed to produce browning of foods which are conventionally expected to have a browned surface, such as bread and other baked products.

The invention has the distinct advantage over prior apparatus and methods in the field of microwave food processing or other heat processing that relatively uniform heating occurs throughout products of widely varying sizes and shapes while the life expectancies of the sources of the microwaves of different frequencies, especially the source of microwaves of lower frequency, are substantially increased due to the substantial reduction in feeback of microwave energy of higher frequency into the source of microwave energy of lower frequency employed in the food processing or other heat processing equipment.

It will be understood that various changes in the details, materials and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art, within the principle and scope of the invention as expressed in the appended claims.

We claim:

1. Microwave heating apparatus comprising an oven which comprises six walls defining a rectangular parallelepiped, a first one of said six walls having a first source of microwave energy of a frequency f, coupled thereto and disposed to radiate microwaves of frequency f, from said first wall into said oven, a second of said six walls having a second source of microwave energy of a frequency f coupled thereto and disposed to radiate microwaves of frequency f from said second wall into said oven, said first and second walls being substantially perpendicular to each other, said frequency f having a value more than twice the value of frequency f whereby said apparatus when operative is substantially free of feedback of said microwaves of frequency f into said first source of microwave energy of frequency f,.

2. Microwave heating apparatus according to claim 1, wherein J"; has a value of about 915 MHz. and f has a value of about 2,450 MHz.

3. A method of heat treating a material having a relatively high dielectric constant disposed within an oven, comprising irradiating said material with microwave energy of two different frequencies at the same time,

tially free of feedback into the source of the other of said different frequencies.

4. A method of heat treating a material in accordance with claim 3, wherein one of said different frequencies has a value of about 915 MHz. and the other of said different frequencies has a value of about 2,450 MHz.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,806,689 Dated A il 23 1971;

Inventofls) James E. Kegerreis. John '1. Perry, Jr. and Ernest M. Kenyon It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

. The frgntjage of the patent, under United States Patent 12- and under 72/, "Kegereis" should read --Kegerreis--.

Column 1, line 22, "brewing" should read --browning--; line 6 4, "magnetrans" should read --magnetrons-. Column 4, line 35, "feeback" should read --feedba.ck--.

Signed and sealed this 3rd day of September 1974.

( SEAL) Attest: MCCOY M. GIBSON, JR. (3. MARSHALL DANN Attesting Officer Commissioner of Patents F ORM PO- 1050 (10-69) UsCOMM-DC 60376-P69 w 0.5 sovzmmzu'r PRINTING arm: 1 I!" o-au-su UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION T 3,806,639 Dated April 23, 197

Inventor( James E. Kegerreis, John T. Perry, Jr. i and Ernest M. Kerggm It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The frgntpage of the patent, under United States Patent l and under L 7 5 "Kegereis" should read --Kegerreis--.

Column 1, line 22, "browing" should read --browning--; line 6 4-, ma.gnetra.ns" should read --ma.gnetrons--. Column 4, line 35, "fee'back" should read "feedback".

Signed and sealed this 3rd day of September 1974.

(SEAL) Attest:

MoCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents FORM P0 1050 (10-69) USCOMM-DC 60376-P69 w u.s. GOVERNMENT rmumm orllc: I!" o-au-su, 

1. Microwave heating apparatus comprising an oven which comprises six walls defining a rectangular parallelepiped, a first one of said six walls having a first source of microwave energy of a frequency f1 coupled thereto and disposed to radiate microwaves of frequency f1 from said first wall into said oven, a second of said six walls having a second source of microwave energy of a frequency f2 coupled thereto and disposed to radiate microwaves of frequency f2 from said second wall into said oven, said first and second walls being substantially perpendicular to each other, said frequency f2 having a value more than twice the value of frequency f1 , whereby said apparatus when operative is substantially free of feedback of said microwaves of frequency f2 into said first source of microwave energy of frequency f1.
 2. Microwave heating apparatus according to claim 1, wherein f1 has a value of about 915 MHz. and f2 has a value of about 2,450 MHz.
 3. A method of heat treating a material having a relatively high dielectric constant disposed within an oven, comprising irradiating said material with microwave energy of two different frequencies at the same time, one of said frequencies having a value more than twice that of the other of said frequencies, the microwaves of each of said different frequencies being coupled to said oven through a sidewall of said oven which is substantially perpendicular to the sidewall through which said other different frequency is coupled to said oven, whereby each of said different frequencies is substantially free of feedback into the source of the other of said different frequencies.
 4. A method of heat treating a material in accordance with claim 3, wherein one of said different frequencies has a value of about 915 MHz. and the other of said different frequencies has a value of about 2,450 MHz. 